There are many known pearlescent or nacreous pigments which are based on micaeous or other lamellar substrates which have been coated with a metal oxide layer. As a result of reflection and refraction of light, these pigments exhibit a pearl-like luster. Depending on the thickness of the metal oxide layer, they can also exhibit interference color effects. A good description of this type of pigment can be found in U.S. Pat. Nos. 3,087,828 and 3,087,829.
The pearlescent pigments most frequently encountered on a commercial basis are titanium dioxide-coated mica and iron oxide-coated mica pearlescent pigments. It is also well-known that the metal oxide layer may be over-coated. For instance, said U.S. Pat. No. 3,087,828 describes the depositing Fe.sub.2 O.sub.3 onto a TiO.sub.2 layer while U.S. Pat. No. 3,711,308 describes a pigment in which there is a mixed layer of titanium and iron oxides on the mica that is overcoated with titanium dioxide and/or zirconium dioxide.
The oxide coating is in the form of a thin film deposited on the surfaces of the mica particle. The resulting pigment has the optical properties of thin films and thus the color reflected by the pigment arises from light interference which is dependent on the thickness of the coating. Since iron oxide has an inherent red color, a mica coated with this oxide has both a reflection color and an absorption color, the former from interference, the latter from absorption of light. The reflection colors range from yellow to red and the pigments are generally referred to as "bronze", "copper", "russet", etc. The pigments are used for many purposes such as incorporation in plastics and cosmetics as well as outdoor applications such as automotive paints.
Pearlescent pigments containing ferrites are also known. For example, U.S. Pat. No. 5,344,488 and DE 4120747 describe the deposition of zinc oxide onto mica platelets which had been coated with iron oxide. The U.S. patent states that in order to avoid the disadvantage of conventional zinc oxide/mica pigments, namely the tendency to agglomerate, and to obtain a pigment which had good skin compatibility, anti-bacterial action, favorable optical absorption properties and a surface color, the zinc oxide layer is applied to a previously prepared metal oxide-coated plate-like substrate. When calcined, small needle shaped crystallites are randomly distributed on the surface layer so that the zinc ferrite layer obtained is not entirely continuous. The patent states that unlike substrates covered entirely with zinc oxide in a continuous layer, the substrates covered with a layer containing crystallites show only a slight tendency to agglomeration.
The quality of a pearlescent pigment is generally dependent upon the smoothness or continuousness of the coating on the micaceous substrate. The quality of the pigment decreases rapidly with increasing discontinuities in the coating. Said U.S. Pat. No. 5,344,488 indicates that discontinuity is essential in order to avoid agglomeration. Therefore, quality must be sacrificed in order to obtain a usable pigment.
It is the object of the present invention to provide a ferrite coated micaceous pigment which is substantially free of needle-shaped crystallites and, therefore, has high quality. This and other objects of the invention will become apparent to those of ordinarily skill in this art.